The present application aims at using neuroimmune approaches to understand the neurobiological basis of behavior. The objective is to study how dysregulated interactions between the nervous and immune systems contribute to comorbidity of depression and pain. Depressive disorders and chronic pain represent two major health burdens in the Western world. Chronic pain predisposes to depression and vice versa, and 30-60% of the cases suffer from both depression and chronic pain. The broad question we seek to answer using a neuroimmune approach is: Why is chronic pain a risk factor for depression, and vice versa? Peripheral inflammation causes sickness behavior that can culminate into depressive behavior when the tryptophan metabolizing enzyme 2,3 indoleamine dioxygenase (IDO) is upregulated. We recently made the exciting discovery that an intracellular protein known as G protein coupled receptor kinase 2 (GRK2) possesses anti- inflammatory properties and acts as a molecular switch that regulates transition from acute to chronic pain. Our newest preliminary data indicate that low GRK2 enhances IDO expression by microglia in vitro. This proposal will test the completely novel concept that GRK2 is a critical molecule that explains the shared risk for developing depression and chronic pain. We have shown that chronic neuropathic pain or inflammation significantly reduces GRK2 in microglia from rodents. This finding is clinically important because low GRK2 in microglia is sufficient to transform transient inflammatory pain into chronic pain. We also showed that low GRK2 augments pro-inflammatory cytokine production and increases activation of p38 in vivo and in vitro. Moreover, we have preliminary data that p38 activity regulates IDO expression, which is key to development of inflammation-associated depressive-like behavior. We hypothesize that the reduction in microglial GRK2 caused by chronic neuropathic pain increases microglial p38 activity, pro-inflammatory cytokine production and IDO expression, thereby acting as risk factor for prolonged depressive-like and pain behaviors. To test this hypothesis, we will answer the following specific questions: 1. Is development of neuropathic pain and depressive-like behavior temporally related to the inflammation-induced reduction in GRK2 and the increase in IDO in CNS microglia/mF? 2. Is low GRK2 a risk factor for development of depressive-like behaviors and what is the mechanism? We will use mice with low GRK2 in microglia that we have generated using Cre-Lox technology. 3. Are comorbid depression and chronic pain both prevented by treatments that interfere with the loop of reduced GRK2, increase in p38 activity and upregulation of IDO? Collectively, these innovative experiments will identify low GRK2 as a completely novel risk factor for development of comorbid depression and chronic pain via a p38/cytokine/IDO-dependent pathway. Identification of these new molecular mechanisms underlying comorbid depression and pain is needed for effective development of novel prevention and therapeutic strategies.